The common adverse reactions associated with diphtheria, tetanus and pertussis (DTP) vaccination are less frequent and less severe with acellular DTP (DTaP) vaccines than with whole cell DTP (DTwP) vaccines. Five key parameters have been identified whose prevalence through the first 48 hours following vaccination accurately characterise the common manifestations of DTaP and DTwP reactogenicity: fever > 37.8 degrees C, moderate or severe fussiness, injection site redness, injection site swelling, and moderate or severe injection site pain. For every DTaP vaccine evaluated, significantly less reactogenicity has been observed in comparison to DTwP vaccines. Although there are differences among the acellular DTaP vaccines in the frequency and severity of some of the adverse reactions, these differences are minor in degree and nature. For both DTaP and DTwP vaccines, local reactions increase in prevalence and severity with successive inoculations. The safety of DTaP vaccines has now been established in infants receiving inoculations at 2,4 and 6 months of age and in other 2 and 3 inoculation sequences below 1 year of age, as well as in toddlers (1 to 3 year olds), preschool children (4 to 6 year olds) and in adults. The increased purity of DTaP vaccines should lead to a reduction in the rare, serious adverse reactions associated with pertussis vaccination.